Means for controlling a forced commutated ac-to-dc electric rectifying circuit to avoid commutation failure

ABSTRACT

A phase-controlled ac-to-dc rectifying circuit includes a pair of alternately conducting main controllable electric valves and cyclically operative forced commutation means for turning off the main valves. The forced commutation means comprises a pair of auxiliary controlled rectifiers respectively connected, in circuit with a commutating capacitor, across the main valves, and the auxiliary rectifiers are alternately turned on in response to periodic trigger signals that are normally produced by associated control means. Protective means is provided for inhibiting the trigger signals when the magnitude of voltage across the commutating capacitor is lower than a critical level which depends on the magnitude of the current being conducted by the main valves, thereby avoiding a risk of commutation failure due to low capacitor voltage.

BACKGROUND OF THE INVENTION

The present invention relates generally to an electric power converter comprising controlled electric valves and forced commutation means for turning off the valves, and it relates more particularly to an improved feature of the controls for a converter well suited to supply direct current (d-c) of variable magnitude to the d-c traction motors on an electric locomotive that is energized from a wayside source of alternating voltage.

Large electrically driven land vehicles such as locomotives and transit cars are propelled by a plurality of traction motors whose rotors are mechanically coupled though speed-reduction gearing to the respective axle-wheel sets of the vehicle. Such motors are usually of the d-c type. If the vehicle is intended to travel along an electrified right of way, it is equipped with a current collector (e.g., a pneumatic or spring-loaded pantograph on the roof of the vehicle) that makes sliding or rolling contact with a bare conductor (e.g., an overhead catenary) extending parallel to the rails or guideway defining the traction vehicle's path of movement. The overhead conductor is part of a power distribution system comprising a plurality of spaced-apart substations which in turn are fed from a stationary source of high-voltage electric power. In practice the power source typically is a commercial power generating plant that supplies 3-phase alternating current (a-c) at a standard frequency such as 60 Hz in the United States or 50 Hz in Europe.

In order to convert the a-c power that is available from the wayside distribution system into direct current of variable magnitude suitable for energizing the armature and field windings of the d-c traction motors on the vehicle, the propulsion system includes a voltage step-down power transformer and a controllable electric power converter. The transformer comprises a single-phase, high-voltage primary winding connected between the current collector and the vehicle wheels (which are at ground potential) and a plurality of lower voltage secondary windings. The converter comprises two or more single-phase, full-wave rectifying bridges. Each bridge has two load-current carrying branches or legs connected in parallel with one another between a pair of output terminals which are connected to at least one of the traction motors, and each leg of the bridge comprises at least one pair of serially connected unilaterally conducting electric valves. A transformer secondary winding is connected across the junctions of the respective valve pairs in the two legs of each bridge, thereby applying alternating voltage to the bridge. The bridge is effective to convert the applied alternating voltage to direct voltage at its output terminals.

The valves in at least one leg of each rectifying bridge in the propulsion system of an electric locomotive are usually uncontrolled devices or simple diodes. In order to vary the output voltage of at least one of the bridges, either the other leg of that bridge comprises a pair of serially connected, periodically conducting controllable valves (thereby forming a semi-controlled asymmetrical or "hybrid" rectifying circuit), or the bridge includes an inverse-parallel pair of periodically conducting controllable vales in the a-c connection between the transformer secondary and the juncture of an uncontrolled valve pair. The average magnitude of the output voltage can then be varied as desired by suitably varying, in synchronism with the applied voltage, the "conduction angles" of such controllable valves (i.e., the lengths of their conducting periods, measured in electrical degrees). During intervals when neither of the controllable valves is in a conducting state, there is no current in the associated secondary winding and motor current will coast or "free-wheel" through a diode leg of the rectifying bridge.

For maximum efficiency the controls of the propulsion system on a vehicle such as an electric locomotive are conventionally designed to work the traction motors at substantially constant horsepower throughout a wide speed range of the locomotive. This is usually done by regulating the magnitude of motor current (and hence motor torque) so that it varies inversely with approximately the square root of locomotive speed as the latter varies between a predetermined "corner point" speed and rated maximum speed. But from zero speed to the corner point speed, maximum current is desired in order to provide high tractive force or effort for accelerating the locomotive from rest. In this low speed range, the voltage applied to the armature windings of the traction motors is relatively low because the counter emf of each motor, which is proportional to speed, is relatively low. At maximum speed maximum voltage must be applied to the motors to overcome their high counter emf, while the magnitude of motor current can now be relatively low because the motors draw less current at high speed that at low speed.

To meet the above-reviewed requirements of both low speed and maximum speed propulsion, it has heretofore been common practice to "stage" the rectifying bridges so that initially, as the locomotive is accelerating from rest, the direct voltage applied to a traction motor is provided by a single bridge (which includes controllable valves) and so that later, when the locomotive attains full speed, the motor voltage is the sum of voltages individually contributed by two or more rectifying bridges whose output terminals are interconnected in series. One well known way to do this is to use at least first and second hybrid bridges in series and to control them in sequence so that during the first stage the conduction angle of each controllable valve in the first bridge is continuously increased from zero to maximum (approximately 180 electrical degrees) while motor current passes through the diode leg(s) of the other bridge(s), thereby increasing the average magnitude of motor voltage from zero to a level equalling the maximum output voltage of the first bridge, whereas during a second stage the conduction angle of each controllable valve in the second bridge is similarly increased while maintaining a maximum conduction angle in the first bridge, thereby further increasing motor voltage to a higher level which equals the maximum output voltage of the first bridge plus the output voltage of the second bridge.

In the present state of the art, the main load-current carrying electric valves in the controllable leg of the hybrid rectifying bridge are high-power, solid-state controllable switching devices known as thyristors or semiconductor controlled rectifiers (SCRs). A thyristor is typically a three-electrode device having an anode, a cathode, and a control or gate terminal. When its anode and cathode are externally connected in series with an electric power load and a source of forward anode voltage (i.e., anode potential is positive with respect to cathode), a thyristor will ordinarily block appreciable load current until a firing or trigger signal is applied to the control terminal, whereupon it switches from its blocking or "off" state to a conducting or "on" state in which the ohmic value of the anode-to-cathode resistance is very low. The time at which the thyristor is turned on, measured in electrical degrees from a cyclically recurring instant at which its anode voltage becomes positive with respect to its cathode at the start of the appropriate half cycle of alternating voltage applied to the bridge, is known as the "firing angle." The average magnitude of the output voltage of a hybrid bridge can be varied by retarding or advancing the firing angle as desired. This is popularly known as "phase control." Hereinafter the firing angle is sometimes also referred to as the "ignition angle."

Once a thyristor is turned on, it can be turned off only by reducing its current below a given holding level and applying a reverse voltage across the anode and cathode for a time period sufficient to allow the thyristor to regain its forward blocking ability. In a conventional line-voltage commutated phase-controlled hybrid bridge, a conducting thyristor is naturally turned off at the end of each half cycle of the applied voltage, at which time the corresponding diode in the parallel diode leg of the bridge becomes forward biased and current transfers from the thyristor to the diode. Such current transfer is referred to as commutation, and the length of the commutation interval (when both the outgoing or relieved thyristor and the incoming or relieving diode are simultaneously conducting) will depend on the magnitude of current being commutated and the inductance (including the leakage reactance of the transformer secondary winding) in the paths of changing current.

Phase-controlled rectifier operation imposes a lagging power factor load on the a-c source. In other words, when the ignition angle of the thyristor leg in a conventional hybrid bridge is delayed or retarded from a fully advanced condition, the fundamental component of alternating current in the transformer primary winding tends to lag the fundamental component of the source voltage, and the locomotive draws undesired reactive or apparent power in addition to useful real power from the wayside distribution system. This is particularly objectionable when the propulsion system is in a high, constant horsepower mode and the locomotive is traveling through a region of the electrified right of way that is relatively remote from the nearest wayside substation. To reduce the reactive component of power and consequently to improve the power factor of an electric locomotive (i.e., to obtain a power factor that approaches unity), it has heretofore been proposed to provide means for periodically turning off the main thyristor leg of one of the hybrid rectifying bridges prior to the time at which commutation would otherwise naturally occur. For this purpose, turn off means known as a forced commutation circuit can be used. Such means enables the conducting thyristor to be quenched or turned off at any desired "extinction angle." The extinction angle, which is measured in electrical degrees from the cyclically recurring negative-to-positive zero crossing the the anode voltage on the main thyristor, marks the time at which suitable action is initiated to turn off the main thyristor and thereby extinguish its current. By appropriately controlling both the ignition angle and the extinction angle of the thyristors in one bridge, the average magnitude of the net output voltage that is applied to the traction motors can be varied as desired while the fundamental component of transformer primary current is positioned to remain nearly in phase with the fundamental component of primary voltage.

A typical forced commutation circuit comprises at least one pair of auxiliary thyristors connected in circuit with at least one commutating capacitor across the respective main thyristors of the rectifying bridge so that each main thyristor in turn is forced to turn off by triggering a complementary one of the auxiliary thyristors. In operation, upon turning on the appropriate auxiliary thyristor the voltage on the precharged commutating capacitor is connected across the then conducting main thyristor with reverse polarity, and load current is quickly diverted from the main thyristor to a parallel path provided by the commutating capacitor and auxiliary thyristor. Thus current is transferred or commutated from the main thyristor to the auxiliary thyristor. After its current is reduced to zero, the main thyristor is temporarily reverse biased by the voltage across the discharging commutating capacitor, and it regains its forward blocking ability during this reverse bias or turn off interval. The extinction angle of the main thyristor therefore coincides with the firing of the auxiliary thyristor.

The aforesaid turn off interval ends when the commutating capacitor is fully discharged. Thereafter load current recharges the commutating capacitor with opposite polarity, and forward anode voltage is reapplied to the turned off main thyristor. The capacitor voltage will soon rise to a magnitude exceeding that of the applied alternating voltage, whereupon the difference therebetween forward biases a previously non-conducting diode in the same rectifying bridge. As a result, load current is now commutated or transferred from the auxiliary thyristor to the diode leg of the bridge. Consequently the auxiliary thyristor turns off and the commutating capacitor is left with a charge of proper polarity for subsequently forcing the other main thyristor to turn off when its complementary auxiliary thyristor is triggered during the succeeding half cycle of operation.

The principle of using forced commutation to obtain unity power factor is further explained in prior art U.S. Pat. No. 3,392,319. Various forced commutation circuits are known, as illustrated, for example, by U.S. Pat. Nos. 3,849,718 and 4,181,932. The ability to control the turn off of the main valves in one of the rectifying bridges of an electric locomotive enables the locomotive to be operated at a desirably high power factor. A high power factor results in lower power losses and less voltage droop, thereby reducing the required size and cost of not only the power transformer on board the locomotive but also the transformers and generators in the wayside electric power system. However, designing a locomotive propulsion system using forced commutation to improve its power factor has necessitated finding solutions to several practical problems.

One problem with a forced commutation circuit is the possibility that the charge on the commutating capacitor can fall below a critical minimum level required for successful commutation. In a forced commutation circuit of the type having only one commutating capacitor of a given size, the available turn off interval tends to vary directly with the magnitude of capacitor voltage and inversely with the magnitude of load current. If the turn off interval were less than a predetermined minimum which equals the specified turn off time of the particular main thyristors that are being used in the rectifying bridge (e.g., 300 microseconds), there is a possibility that the outgoing main thyristor may not have fully recovered its forward blocking ability at the time forward voltage is reapplied to this device, in which event the commutation circuit would malfunction and the thyristor could fail. It is therefore unwise to attempt to turn off a main thyristor under marginal conditions when the attempt might be aborted.

The problem of avoiding a commutation failure due to insufficient energy being stored in the commutating capacitor is addressed in U.S. Pat. No. 3,659,119 which discloses a forced commutation thyristor 37 chopper" in which both the instantaneous magnitude of load current in the main thyristor and the magnitude of voltage across the commutating capacitor are detected and the auxiliary thyristor is fired whenever current reaches a prescribed level which varies as a predetermined function of voltage. In other words, for any given magnitude of capacitor voltage, commutation will automatically be initiated if and when load current attains a critical value that does not exceed the maximum limits of safe commutation. Thus load current is kept within the extinguishing ability of the commutating capacitor. While this scheme may be technically feasible in the controls of a dc-to-dc chopper, it would result in unacceptable variations of the extinction angle and could result in load current (and hence tractive effort) being undesirably limited in the practical application of a forced commutated ac-to-dc rectifying bridge herein contemplated.

SUMMARY OF THE INVENTION

A general objective of the present invention is to provide new and improved means for protecting a forced commutated rectifying circuit from commutation failure if and when the commutating capacitor has insufficient voltage for safe operation.

Another objective of the invention is to control a forced commutated phase controlled rectifying circuit in a manner that avoids commutation failure without limiting load current as a function of the voltage on the commutating capacitor.

In carrying out the invention in one form, an electric load circuit such as a plurality of traction motors on an electric locomotive is supplied with direct current by a converter comprising a rectifying bridge having a pair of d-c output terminals to which the load circuit is connected, a-c input terminals adapted to be connected to a source of alternating voltage, and controllable means for interconnecting the input and output terminals. The controllable means include at least one pair of alternately conducting main electric valves having cyclically operative valve turn off means respectively associated therewith. The valve turn off means is a forced commutation circuit comprising at least one pair of auxiliary controlled rectifiers connected, in circuit with a precharged commutating capacitor, across the respective main valves, whereby each of the main valves in turn is forced to turn off upon turn on of a complementary one of the auxiliary rectifiers. Turn on control means is provided for generating a set of firing signals capable of periodically turning on the main valves at an ignition angle that can be advanced or retarded as desired, and turn off control means is provided to produce periodic trigger signals for turning on the auxiliary rectifiers so that normally the valves are turned off at a desired extinction angle which varies within predetermined limits. With both the ignition and the extinction angles being suitably controlled, the converter is able to supply the d-c load circuit with current of desired magnitude without imposing a load of undesirably low power factor on the a-c source.

Associated with the converter and its control means are means for providing voltage and current feedback signals representative, respectively, of the voltage across the commutating capacitor and of the magnitude of current supplied to the d-c load circuit. Protective means responsive to these feedback signals is provided for preventing the production of the aforesaid trigger signals anytime the magnitude of capacitor voltage is lower than a critical level determined by the magnitude of current, whereby normal valve turn off operation of the turn off control means is inhibited if capacitor voltage is insufficient to ensure successful forced commutation of the load current being conducted by the main valves of the rectifying bridge. By thus preventing forced commutation when the voltage on the commutating capacitor is relatively low, the risk of valve failure that attends such a marginal condition is avoided, at the cost of poor power factor while such condition prevails. In practice, low voltage is a transient or temporary condition, and the protective means will permit normal operation of the valve turn off control means to resume as soon as sufficient capacitor voltage is restored.

The invention will be better understood and its various objects and advantages will be more fully appreciated from the following description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram of part of an electric propulsion system on board a vehicle such as an electric locomotive, the illustrated part of the system comprising a single-phase transformer and three serially interconnected hybrid rectifying bridges that convert wayside a-c electric power to d-c power for energizing one group of traction motors on the vehicle, with the first one of the three bridges including a forced commutation circuit for turning off the main valves in the controlled leg of this bridge;

FIGS. 1A and 1B are functional block diagrams of means for providing feedback signals representative of traction motor speed and current, respectively;

FIG. 2 is a functional block diagram of the controls for the rectifying bridges shown in FIG. 1, including the improvement of the present invention for controlling the turn off of the controllable main valves in the first bridge;

FIG. 3 is an expanded functional block diagram of the firing synchronizing and pulse generator block of FIG. 2;

FIG. 4 is a schematic diagram of the "β logic" circuit shown as a single block in FIG. 3, the output of this circuit comprising trigger signals which periodically activate the turn off gate pulse generator block labeled "#4 GPG" in FIG. 3; and

FIG. 5 is a chart illustrating how voltage and current vary with time in certain components of the first rectifying bridge during a forced commutation interval.

DETAILED DESCRIPTION

FIG. 1 depicts an electric propulsion system comprising a group of three d-c traction motors M1, M2, and M3 suitable for propelling or retarding a large traction vehicle such as a locomotive or transit car. Although not shown in the drawings, it will be understood that the rotors of these motors are respectively coupled by speed-reduction gearing to separate axle-wheel sets of one truck of the locomotive. A 3-axle truck has been assumed; only two motors would be used on a 2-axle truck. A second group of three (or two) duplicate motors which are associated with the other truck of the locomotive has been omitted in FIG. 1 for the sake of drawing simplicity.

The motors M1, M2, and M3 are respectively provided with field windings 11, 12 and 13. In the illustrated embodiment of the invention, these field windings are in series with the respective armature windings of the motors whenever the propulsion system is in its motoring mode of operation. The motors are connected in parallel with one another across a pair of relatively positive and negative d-c power conductors 14p and 14n, respectively. As is shown in FIG. 1, conventional isolation contactors 15, 16 and 17, current smoothing reactors 18, 19, and 20, and suitable current transducers 21, 22, and 23 are respectively connected in series with the three motors between the d-c conductors. The current transducers 21, 22, and 23 provide signals I1, I2, and I3 proportional to the magnitudes of current in the armatures of the motors M1, M2, and M3, respectively. Three speed sensors 24, 25, and 26 of conventional design are respectively coupled to the rotors of the motors or to the axle-wheel sets of the locomotive truck to provide signals W1, W2, and W3 having frequencies that vary with the angular velocities of the respective motor armatures. Assuming good adhesion between wheels and track, locomotive speed (miles or kilometers per hour) is proportional to motor speed (revolutions per minute). Assuming equal adhesion and equal wheel diameters, the three signals W1, W2, and W3 will have substantially equal values.

As can be seen in FIG. 1A, the signals W1, W2, and W3 from the three speed sensors are fed to a maximum value selector 27 which produces on a line 28 a speed feedback signal S representative of the highest speed of the three motors M1, M2, and M3. The value of S will vary between 0 when the locomotive is at rest and 1.0 per unit when the locomotive is moving at its maximum rated speed (e.g., 75 mph). The speed feedback signal S is supplied to the input of a speed threshold detector 29 which has two output lines 30 and 31. The detector 29 is suitably designed and constructed to produce an output signal SL on the line 30 in response to speed attaining a relatively low threshold (e.g., approximately 0.07 per unit) and to produce an output signal SH on the line 31 in response to speed attaining a predetermined high threshold (e.g., approximately 0.40 per unit). In the illustrated embodiment of the invention, each of the output signals SL and SH is a digital logic signal, i.e., it is low or " 0" so long as S has a value indicating that motor speed is lower than the designated threshold, whereas it is high or "1" whenever S indicates motor speed equals or exceeds that threshold.

As can be seen in FIG. 1B, the signals I1, I2, and I3 from the three current transducers are fed to a maximum value selector 32 which produces on a line 33 a current feedback signal I representative of the magnitude of direct current in whichever one of the three motors M1, M2, and M3 has the highest current. The current feedback signal I is supplied to the input of a current threshold detector 34 which is suitably designed and constructed to produce an output signal IL on a line 35 in response to the magnitude of motor current attaining a predetermined low level (e.g., 400 amps). The output signal IL is low or "0" so long as I has a value indicating that the magnitude of motor current is lower than such a level, whereas it is high or "1" whenever I indicates motor current equals or exceeds that level.

Returning now to FIG. 1, the d-c power conductors 14p and 14n are coupled to the secondary of a power transformer 38 via an electric power converter comprising three full-wave rectifying circuits 1, 2 and 3. The power transformer has a single-phase primary winding 40 and a plurality of secondary windings, including first, second and third windings 41, 42, and 43, respectively. One terminal of the transformer primary winding 40 is connected to a path 44 at ground potential (e.g., the platform, axles, and wheels of the locomotive and the rails of the track on which the locomotive travels), and the other terminal of winding 40 is connected via the separable main contacts 45 of a power circuit breaker (preferably of the vacuum breaker type) to a suitable current collector 46 on the vehicle (e.g., a pantograph) that maintains sliding contact with an overhead conductor 47 (e.g., a catenary). The catenary 47 is part of a wayside a-c power distribution system that supplies alternating current to the locomotive from a stationary source 48 at substantially constant frequency and relatively high voltage with respect to ground (e.g., 25,000 volts rms at the catenary sections that adjoin wayside power substations). The transformer 38 steps down the source voltage to single-phase alternating voltages V1, V2, and V3 of lower magnitude (e.g., 555 volts rms) on the secondary windings 41, 42, and 43, respectively. These voltages are substantially in phase with one another. While only one set of three secondary windings are shown in FIG. 1, in practice additional secondaries would ordinarily be used.

The three rectifying circuits 1, 2, and 3 shown in FIG. 1 are hybrid or semi-controlled asymmetrical bridges. Each of them has first and second legs connected in parallel with one another between a pair of d-c output terminals. The first leg of each bridge comprises at least one pair of serially connected uncontrolled electric valves D1p and D1n, whereas the second leg comprises at least one pair of serially connected, alternately conducting controllable main electric valves T1n and T1p. All of these valves are polled to conduct current in a direction from the relatively negative to the relatively positive d-c output terminal. In the presently preferred embodiment of the invention, the uncontrolled valves are solid-state power rectifier diodes, and the controllable valves are solid-state controlled rectifiers or thyristors. While each such device has been shown and described in the singular, in practice it can actually comprise a combination of two or more duplicate devices that are interconnected in parallel (to increase the forward current rating of the leg) and/or in series (to increase the reverse voltage rating).

As can be seen in FIG. 1, the three secondary windings 41, 42, and 43 of the power transformer are respectively connected between the juncture of the diodes D₋₋ p and D₋₋ n in the first legs of the respective bridges and, via current limiting fuses 51, 52, and 53, respectively, the juncture of the thyristors T₋₋ n and T₋₋ p in the second legs of the respective bridges, whereby in-phase alternating voltages are respectively applied to corresponding a-c input terminals of the three bridges 1, 2 and 3. The d-c terminal pairs of the bridges are connected in series with one another, whereby the output voltage V_(o) of the converter is the sum of the direct voltages at the individual outputs of the respective bridges 1, 2, and 3. This net output voltage is applied to the load circuit comprising the d-c power conductors 14p and 14n, the armature and series field windings of the parallel array of traction motors M1, M2, and M3, and the reactors 18-20 that are in series with the respective motors. By suitably controlling the thyristor legs of the separate bridges, the average magnitude of V_(o) can be varied as desired.

Each of the three bridges 1, 2, and 3 includes appropriate means for producing a fault signal F1, F2, or F3 in the abnormal event of a short circuit in the bridge. Such means can comprise, for example, a conventional "trigger" mechanism associated with each of the current limiting fuses 51, 52, and 53 for indicating if and when the current interrupting element of the fuse is ruptured by the overcurrent that will flow in the a-c connections of the bridge if either one of the associated pair of main thyristors were shorted (i.e., if a thyristor were to fail to withstand reverse voltage). Alternatively, the overcurrent signal can simply comprise the voltage that will be present across the terminals of the fuse once its interrupting element has blown. p In order to reduce the amount of reactive power that the illustrated propulsion system draws from the wayside source 48 and thereby to increase the power factor of the locomotive, means for controlling the extinction angle of the main thyristors T1p and T1n in the first bridge 1 is desired. For this purpose these thyristors have cyclically operative valve turn off means respectively associated therewith. Preferably the valve turn off means is a forced commutation subcircuit 4 comprising at least one pair of auxiliary controlled rectifiers T4a and T4b connected in circuit with a commutating capacitor 54 across the respective main thyristors T1p and T1n in bridge 1. More specifically, the auxiliary rectifiers are serially connected between the negative and positive output terminals of the bridge 1, and the capacitor 54 spans the juncture of these devices and the juncture of the fuse 51 and a line 50 connecting the fuse 51 to the no-dot end of the transformer secondary winding 41. With this arrangement, during a positive half cycle of the applied voltage V1 (i.e., the half cycle during which the dot end of the secondary winding 41 is positive with respect to the line 50), the main thyristor T1p can be forced to turn off by turning on its complementary auxiliary rectifier T4a, and during a negative half cycle of V1 the main thydristor T1n can be forced to turn off by turning on its complementary auxiliary rectifier T4b. Suitable means (not shown) is provided for precharging the commutating capacitor 54 so that prior to any phase control operation of the rectifying bridge 1 the capacitor voltage V4 has sufficient magnitude for successful commutation.

The operation of the bridge 1 will now be briefly summarized for a typical half cycle during which both ignition and extinction angles of the thyristor leg are controlled. Assume that the commutating capacitor 54 is charged so that the electric potential at the juncture of the rectifiers T4a and T4b is negative with respect to the line 50, and also assume that appreciable load current (e.g., 4,500 amps) is flowing through the parallel array of traction motors M1, M2, and M3. At the beginning of a positive half cycle of transformer secondary voltage V1, load current will freewheel through the diode leg D1n, D1p of the bridge 1, there is negligible voltage across the d-c output terminals of this bridge, and V1 is applied as a forward anode-to-cathode bias across the main thyristor T1p which withstands this voltage until its control electrode or gate receives a firing signal at the desired ignition angle. The firing signal turns on T1p, whereupon load current is commutated from diode D1n to thyristor T1p. The rate at which current increases in T1p and decreases in D1n depends on the leakage reactance of the transformer secondary winding 41 and the amount of any other inductance in series with these devices. (While not shown in FIG. 1, it is conventional practice to provide a commutating inductor between the juncture of the main thyristors and the line 50.) During the interval when both D1n and T1p are simultaneously conducting, known as the commutation interval, the voltage applied to the bridge collapses. However, as soon as thyristor current attains the magnitude of load current and the diode D1n stops conducting, the voltage on the output terminals of the bridge 1 rises to a magnitude equal to the instantaneous magnitude of the transformer secondary voltage V1, and the output voltage will then remain equal to V1 throughout the remainder of the conduction angle of the thyristor T1p. The forward voltage drop across T1p is negligible while this device is in its conducting state.

Subsequently, at time t_(o) which coincides with the desired extinction angle, the auxiliary rectifier T4a is turned on and load current will be commutated from T1p to T4a. The resulting current decrease and voltage increase experienced by the main thyristor are respectively shown by solid-line trace I_(T1p) and broken-line trace V_(T1p) in FIG. 5 which displays these parameters in simplified form. The current now flowing in the auxiliary rectifier will initially discharge the commutating capacitor 54 and then recharge it with opposite polarity. The rate of change of capacitor voltage V4 depends on the magnitude of load current. During the capacitor discharging interval (t_(o) -t₂ in FIG. 5), the main thyristor T1p has its current reduced to zero and is then temporarily reverse biased by V4. The period between the time t₁ at which forward current in T1p decays to zero and the time t₂ at which the capacitor is fully discharged (V4 is zero) is known as the circuit turn off interval (T_(OFF)), and during this interval T1p regains its forward blocking ability.

After the turn off interval, the commutating capacitor 54 recharges and its voltage V4 soon rises to a magnitude exceeding the instantaneous magnitude of V1, whereupon the difference between V4 and V1 forward biases the diode D1n. As a result, load current now quickly commutates from the auxiliary rectifier T4a to the diode D1n, and the capacitor 54 is left with a charge of proper polarity and sufficient magnitude for subsequently forcing the other main thyristor T1n to turn off when its complementary auxiliary rectifier T4b is fired at the desired extinction angle during the succeeding negative half cycle of alternating voltage V1. When T4a turns off, V1 is reapplied to the now turned off main thyristor T1p, and load current again freewheels through the diode leg D1n, D1p of the bridge 1 until the other main thyristor T1n is subsequently turned on, at the desired ignition angle, during the negative half cycle of V1.

FIG. 2 depicts the controls that provide properly timed firing signals for the gates of the main thyristors in the three rectifying bridges 1, 2, and 3, respectively, and for the gates of the auxiliary controlled rectifiers in the forced commutation subcircuit 4 of the first bridge 1. A conventional closed loop control system is employed, whereby motor current (and hence tractive effort) is regulated to correspond to a desired magnitude. The desired current magnitude is represented by a current call signal I* which is produced on the output line 55 of a block 56 labeled "decoder and modulator." Inputs to this block are provided by an operator controlled throttle 57 and by a motor speed responsive reference generator 58. The reference generator 58 in turn is connected to the speed feedback signal S on line 28. It is suitably constructed and arranged to supply the decoder and modulator 56 with a maximum current signal having a value which varies as a function of motor speed in the manner indicated by the graph drawn inside the block 58. For any motor speed between zero and a predetermined corner point speed (e.g., nearly 30% of the rated maximum motor speed), the maximum current signal has a constant, relatively high value that results in the propulsion system exerting a relatively high tractive effort, but from corner point speed to maximum speed the value of this signal will decrease with the square root of increasing motor speed so that the horsepower of the propulsion system is high and substantially constant.

The value of the maximum current signal is modulated, according to the given setting of the locomotive throttle 57, by the decoder and modulator 56. The handle of the throttle 57 conventionally has an idle position and eight other discrete positions or power notches, and the particular position that the operator has selected is indicated by a system of relays whose contact positions are decoded in the block 56. The throttle setting is herein referred to as a command signal. At any given motor speed, the value of the current call signal I* on line 55 is incrementally increased as the throttle position is advanced from idle to notch 8. Therefore maximum torque is realized in notch 8. The block 56 includes reset means for automatically reducing I* to a value corresponding to zero current when desired.

As is indicated in FIG. 2, the output line 55 of the block 56 is connected to a rate limit circuit 59 whose output supplies a current reference signal to one input of a summing point 60. The rate limit circuit 59 performs the conventional function of preventing the value of the current reference signal from being changed too fast. By way of example, the maximum rate of increase of the reference signal can be limited to a rate corresponding to 150 amps per second. The summing point 60 has another input which is supplied with the current feedback signal I on the line 33, and its output is an error signal which is representative of the difference between actual and desired magnitudes of motor current. Preferably the quiescent value of the error signal (i.e., its value whenever both the desired and actual magnitudes of current are zero) is slightly negative with respect to a predetermined reference potential. The error signal is supplied via a line 61 to a suitable gain network or amplifier 62 having a proportional plus integral transfer characteristic, whereby a zero steady-state error can be obtained. The amplifier 62 produces a control signal V_(c) which is fed on a line 63 to one of the inputs of a block 65 labeled "firing reference generator." The value of V_(c) varies, within limits, as a function of any difference or error between the current reference and feedback signals. So long as the error signal is "positive" (i.e., so long as the actual current magnitude is less than desired), the amplifier 62 causes V_(c) to vary in a sense approaching a predetermined upper limit. The value of V_(c) determines the ignition angles of the thyristor legs of the respective rectifying bridges 1, 2, and 3, and it will tend to assume whatever value results in reducing the error signal on line 61 to zero. In the illustrated embodiment of the converter controls, V_(c) is an analog signal, and its magnitude can vary between 0 and +10 volts with respect to the predetermined reference potential. Preferably the reference potential is the same as that of a grounded conductor or bus in the d-c control power circuit.

As is shown in FIG. 2, additional inputs to the firing reference generator 65 are provided by the current call signal I* on line 55, the speed feedback signal S on line 28, the low current signal IL on line 35, and the low and high speed threshold signals SL and SH on lines 30 and 31, respectively. In a manner that is shown and described in a related patent application Ser. No. 429,727 filed on Sept. 30, 1982, in the names of P. C. Chandran and E. S. Matulevich and assigned to General Electric Company, the firing reference generator 65 generates first, second, third, and fourth reference signals on four output lines 71, 72, 73, and 74, respectively. The first reference signal on line 71 will determine the ignition angle (α₁) of the thyristor leg of bridge 1, and normally it is variable from 0 to +10 volts as V_(c) varies over a first predetermined range of values extending from a first or low limit (e.g., 0 volts) to a preselected first intermediate value (e.g., 3.33 volts). The second reference signal on line 72 will determine the ignition angle (α₂) of the thyristor leg of bridge 2, and normally it is variable from 0 to +10 volts as V_(c) varies over a second predetermined range of values extending from approximately the aforesaid first intermediate value to a preselected second intermediate value (e.g., 6.67 volts) outside of the aforesaid first range. The third reference signal on line 73 will determine the ignition value (α₃) of the thyristor leg of bridge 3, and normally it is variable from 0 to +10 volts as V_(c) varies over a third predetermined range of values extending from approximately the aforesaid second intermediate value to the aforesaid upper limit (+10 volts). The fourth reference signal on line 74 determines the desired extinction angle (β) of the thyristor leg of bridge 1, and normally it varies, within predetermined limits, as a predetermined function of motor speed.

The output lines 71-74 of the firing reference generator are coupled to a block 75 labeled "firing synchronizing and pulse generator." Suitable means are provided for supplying the block 75 with signals representative of alternating voltages VB1, VB2, and VB3 applied to the bridges 1, 2, and 3, respectively, and with a signal representative of the voltage V4 across the commutating capacitor 54 in the forced commutation subcircuit 4 that is associated with the thyristor leg of bridge 1. The current feedback signal I on line 33 provides an additional input to the block 75. In a manner that is explained in more detail in conjunction with the description of FIG. 3, the firing synchronizing and pulse generator is normally operative to generate first, second, and third sets of periodic firing signals or gate pulses for respectively turning on the pairs of main thyristors in the bridges 1, 2, and 3 in synchronism with the applied voltages at ignition angles that can be advanced or retarded as desired. More particularly, the ignition angle at which the firing signals for bridge 1 are nominally generated will vary from fully retarded to fully advanced as the first reference signal on line 71 varies from 0 to +10 volts, the ignition angle at which the firing signals for bridge 2 are nominally generated will vary from fully retarded to fully advanced as the second reference signal on line 72 varies from 0 to +10 volts, and the ignition angle at which the firing signals for bridge 3 are nominally generated will vary from fully retarded to fully advanced as the third reference signal on line 73 varies from 0 to +10 volts. Thus the three bridges are controlled in sequence. As the control signal V_(c) increases from its first limit to its upper limit, the average magnitude of the converter output voltage is increased in three stages. With V_(c) increasing in the aforesaid first range of values, α₁ decreases while α₂ and α₃ remain in their maximum retarded conditions, and V_(o) equals the output voltage of bridge 1 alone. With V_(c) increasing in the aforesaid second range, α₂ decreases while α.sub. 3 is fully retarded and α₁ is nominally fully advanced, and V_(o) equals the sum of the output voltage of bridges 1 and 2. Finally, with the value of V_(c) increasing in the third range, α₃ decreases while α₂ and α₁ are in maximum advanced conditions, and V_(o) now equals the sum of output voltages from all three bridges.

As will be better understood from the description of FIGS. 3 and 4, the firing synchronizing and pulse generator 75 is also operative to generate firing signals or gate pulses for the auxiliary controlled rectifiers in the forced commutation subcircuit 4 so that normally the main thyristors in bridge 1 are periodically turned off in synchronism with the voltage VB1 at an extinction angle determined by the magnitude of the fourth reference signal on line 74. In this manner the fundamental component of current in the transformer primary 40 is shifted to be more nearly in phase with the fundamental component of primary voltage, whereby the power factor of the locomotive propulsion system is improved.

Normal operation of the firing reference generator 65 and of the companion firing synchronizing and pulse generator 75 is sometimes prevented by the action of a block 76 labeled "derate and inhibit." Inputs to the block 75 are provided by the speed threshold signals SL and SH on lines 30 and 31, respectively, by the thyristor overcurrent signals F1, F2, and F3, and, if desired, by three transformer winding overcurrent signals T1, T2, and T3 which are received from blocks 77, 78, and 79, respectively. The blocks 77-79 represent conventional instantaneous overcurrent protective relays which are respectively coupled, by means of current transformers (not shown), to the secondary windings 41-43 of the power transformer 38, and each of them is effective to produce a signal T1, T2, or T3 in response to abnormally high transformer secondary current caused by the failure of any one of the valves in the diode leg of the associated bridge 1, 2, or 3.

As is shown in FIG. 2, the derate and inhibit block 76 has four output lines 80 through 83 connected to the block 75, and two of these lines (80 and 82) are also connected to the block 65. The block 76 includes logic circuits suitably arranged to produce a derate signal on the output line 80 and inhibit signals on the respective lines 81, 82, and 83 in response to various combinations of the signals that are received at its inputs. The construction and operation of such logic circuits are more fully described and are claimed in a patent application Ser. No. 429,729 filed on Sept. 30, 1982, and assigned to General Electric Company. Briefly summarized, the circuits in block 76 are effective to produce:

(1) a first inhibit signal on the line 81 for preventing the generation of the first set of firing signals for bridge 1 in the event of an overcurrent signal F1 or T1 (indicating a bridge 1 fault);

(2) a second inhibit signal on the line 82 for preventing the generation of the second set of firing signals for bridge 2 and for adjusting the firing reference generator 65 so that the third reference signal on line 73 varies between 0 and +10 volts as the control signal V_(c) varies over its second range of values (a) in the event of an overcurrent signal F2 or T2 (indicating a bridge 2 fault) or (b) whenever there is no bridge 1 fault, and motor speed is lower than the aforesaid low threshold (as indicated by the signal SL being "0");

(3) a derate signal on the line 80 for inhibiting the firing signals for the auxiliary rectifiers in the forced commutation subcircuit 4, for imposing a predetermined minimum limit on α₁ (unless the magnitude of load current is lower than the aforesaid low level, as indicated by the signal IL being ("0") and for imposing the same minimum limit on α₃ in the event of a fault in either bridge 1 or bridge 2; and

(4) a third inhibit signal on the line 83 for preventing the generation of the third set of firing signals for the bridge 3 whenever (a) there are no overcurrent signals, and motor speed is lower than the aforesaid high threshold (as indicated by the signal SH being "0"), or (b) there is an overcurrent signal F3 or T3 (indicating a bridge 3 fault), or (c) there is a fault in bridge 1 or 2 but not both, and there is no fault in bridge 3, and motor speed is lower than its predetermined low threshold.

As is shown in FIG. 3, the firing synchronizing and pulse generator 75 actually comprises three separate synchronizing and gate pulse generators 171, 172 and 173 which, at ignition angles that are respectively determined by the values of the first, second and third reference signals on line 71, 72 and 73, generate the first, second, and third sets of periodic firing signals that are coupled to the gates of the main thyristors T₋₋ p and T₋₋ n in the bridges 1, 2 and 3, respectively. It also comprises a block 174 labeled "β logic" that is fed by the fourth reference signal (β REF) on line 74 and that is interconnected with the #1 generator 171. The block 174 represents a circuit that is suitably constructed and arranged to produce periodic trigger signals which are fed over a line 175 to a block 176 labeled "#4 GPG." In response to the trigger signals on line 175 and to steering signals received from "enable" 1 and 2 outputs of the generator 171, the block 176 generates firing signals that are coupled to the gates of the auxiliary controlled rectifiers T4a and T4b in the forced commutation subcircuit, which signals in effect are turn off signals for the pair of main thyristors in bridge 1.

Circuit details of the #1 synchronizing and gate pulse generator 171 are shown and described in the above-mentioned copending application of Chandran and Matulevich, the disclosure of which is incorporated herein by reference. The associated β logic circuit 174 receives various signals from the generator 171, and the component and circuits in the generator that supply some of these signals will now be briefly summarized. The generator 171 includes appropriate means for producing a train of synchronizing signals or "sync" pulses of relatively short duration that coincide with successive zero crossings of an a-c signal VB1 representative of the alternating voltage applied to bridge 1 (i.e., the voltage between the respective junctures of the diodes D1p and D1n and of the main thyristors T1p and T1n). It also includes a saw-tooth waveform generator for providing a timing signal, labeled "SAW", having a value which increases abruptly to +10 volts in response to each sync pulse and then decreases at a constant rate to zero volts in a half period of VB1. Suitable means in the generator 171 provide alternate high and low digital logic signals at each of a pair of outputs 1 and 2, which signals serve a steering or "enable" purpose; there is a high enable signal at output 1 concurrently with each "positive" half cycle of VB1, whereas there is a high enable signal at output 2 concurrently with each "negative" half cycle of VB1. In addition, the generator 171 includes bistable voltage detecting means for producing a "volt zero" output signal which is low or "0" whenever the magnitude of VB1 is negligible and which is high or "1" whenever the magnitude of VB1 is appreciable.

Details of the presently preferred embodiment of the β logic circuit 174 are shown in FIG. 4. As is illustrated therein, this circuit includes a conventional comparator 215 having two inputs. The saw-tooth voltage signal from the #1 generator 171 is fed via a line 195 to the negative input of this comparator, and the fourth reference signal (β REF) on line 74 from the firing reference generator 65 is fed to the positive input of the comparator 215. Consequently the signal on the output line 216 of the comparator will change from 1 to 0 when the saw-tooth signal on line 195 rises to +10 volts at the start of each half cycle of VB1, and subsequently it will change from 0 to 1 each time a falling saw-tooth signal intersects a reference signal of positive magnitude on line 74 (i.e., at the instant when the value of the saw-tooth signal just equals the value of β REF). So long as the reference signal on line 74 is positive, the lower its magnitude the later the signal on line 216 changes from 0 to 1 in each half cycle of VB1. As the reference signal approaches zero volts, the 0-to-1 change on line 216 occurs at an electrical angle approaching 180 degrees. If the reference signal were negative, the signal on line 197 would remain 0.

The output line 216 from the comparator 215 is coupled to the input line 217 of a monostable multivibrator 218 (whichis shown as a block labeled "one shot") by way of suitable AND logic means 220 (shown symbolically in FIG. 4 by an encircled dot having multiple inputs). The output of the one shot device 218 appears on the output line 175 of the β logic circuit, and this device is operative periodically, in response to successive 0-1 changes of the signal on its input line 217, to produce on the line 175 a train of discrete β trigger signals of short, fixed duration (e.g., 15 microseconds). These trigger signals are supplied to the #4 gate pulse generator 176 (FIG. 3) which comprises suitable means for alternately transmitting appropriate firing signals to the respective auxiliary controlled rectifiers in the forced commutation subcircuit 4 in response to consecutive signals on line 175. As a result, the turn off action of each of the main thyristors T1p and T1n in bridge 1 is initiated at an extinction angle β that nominally coincides with the angle (measured in electrical degrees from the start of each half cycle of VB1) of the 0-to-1 change of the comparator output signal on line 216. It will now be apparent that the lower the value of β REF the later this change occurs in each half cycle of applied voltage (VB1), and hence the desired β varies inversely with the value of β REF. When certain disabling means in the reference generator 65 are not effective to keep β REF at a negative value, this signal has a positive magnitude that decreases with increasing motor speed up to a predetermined speed threshold at which β is clamped at a maximum limit (e.g., 95°), all as is explained more fully in the above-mentioned concurrently filed application of Chandran and Matulevich.

As is shown in FIG. 4, the output line 175 is also connected to a "set" input of a conventional bistable flip flop device 221. This device has a "reset" input connected to a line 187 on which the aforesaid sync pulses are received from the #1 generator 171 (FIG. 3) and a "Q bar" output connected to an output line 209, whereby the output of this device (referred to as "old β") is high or "1" except during recurrent intervals that commence with the production of each β trigger signal and that terminate at the next sync pulse on line 187 (which marks the end of either a positive or negative half cycle of the alternating voltage applied to bridge 1). The line 209 is coupled to the generator 171, and once a β trigger signal is produced on line 175 the resulting O signal on line 209 is effective in the #1 generator to inhibit another α₁ firing signal until the next half cycle.

The AND logic means 220 will prevent the one shot device 218 from responding normally to a 0-to-1 change of the comparator output signal on line 216 if the signal supplied to its second input is 0. So long as any 0 signal is being supplied to the logic means 220, the signal on line 217 will remain 0 and the device 218 cannot produce any β trigger signals on line 175. This inhibits the firing signals for the auxiliary rectifiers in the forced commutation subcircuit 4 and consequently blocks the thyristor turn off operation of the converter controls.

The second input signal supplied to the logic means 220 comprises the inverted output signal of a conventional OR logic circuit 223 shown symbolically in FIG. 4 by an encircled plus sign. The output of this circuit is connected to an input of the logic means 220 via a signal inverter 224 and a line 225. The signal on line 225 is 0 so long as there is a 1 signal on either of the two input lines 226 and 227 of the circuit 223, and it is 1 only when the signals on both input lines are concurrently 0. The first input line 226 is supplied with the inverted output signal of an AND logic circuit 228 to which is is connected via a signal inverter 229. The two inputs of the latter circuit are connected to lines 80 and 206, respectively. The output signal of the logic circuit 228 will be 0 (and the signal on line 226 will therefore be 1) so long as there is a 0 signal on either line 80 or line 206; otherwise it is 1. As a result, a 0 signal is maintained on the input line 225 of the logic means 220 and normal operation of the one shot 218 is prevented whenever the derate signal on line 80 is 0 (as is the case when the derate and inhibit block 76 detects a fault in either bridge 1 or bridge 2) or whenever the signal on line 206 is 0. The latter signal (referred to as "old α") is received from the #1 generator 171 (FIG. 3), and it will be 0 during recurrent intervals commencing at the start of each half cycle of the alternating voltage applied to the a-c terminals of bridge 1 and terminating in response to the next turn on of thyristor T1p or T1n in the same bridge.

The second input line 227 to the OR logic circuit 223 is supplied with the output signal of a companion OR logic circuit 230 having two input lines 231 and 232. As can be seen in FIG. 4, line 232 is connected to the output of a block 235 labeled "Δt" which in turn receives as inputs the volt zero signal supplied by the generator 171 on a line 181, the sync pulses on line 187, and the inverted old α signal on line 226. The component 235 comprises a time delay circuit suitably constructed and arranged to provide a periodic 1 signal on line 232 (and hence a 0 signal on line 225) until a short time after each turn on of a main thyristor in bridge 1 so as temporarily to extend the aforesaid recurrent intervals during which normal thyristor turn off operation of the β logic circuit is blocked, whereby each of the diodes D1p and D1n in bridge 1 has sufficient time to fully recover its reverse blocking capability after load current is commutated to the corresponding main thyristor before the complementary auxiliary rectifier can be fired. This feature of the β logic circuit is the claimed subject matter of a patent application Ser. No. 429,728 filed on Sept. 30, 1982, in the names of H. J. Brown, I. N. Horvat, and A. K. Kumar and assigned to General Electric Company.

In accordance with the present invention, the input line 231 of the OR logic circuit 230 is supplied with the output signal of a protective circuit 233 comprising an AND logic circuit 252 having first and second input lines 253 and 254, respectively. As is shown FIG. 4, the first input line 253 is supplied with the inverted output signal of an AND logic circuit 255 to which it is connected via a signal inverter 256, whereas the second input line 254 is supplied with the inverted output signal of another AND logic circuit 257 to which it is connected via a signal inverter 258. The circuit 255 has two input lines 191 and 261; line 191 is connected to the #1 generator and 171 (FIG. 3) where it is supplied with the aforesaid enable 1 signal, and line 261 is connected to the output of a first comparator 263. The output signal of the logic circuit 255 will be 1 (and the signal on line 253 will therefore be 0) only when there are concurrent 1 signals on both of the lines 191 and 261; otherwise it is 0. Consequently a 0 signal is maintained on the the input line 231 of the OR logic means 230 so long as the first comparator 263 provides a 1 output on the line 261 during "positive" half cycles of the alternating voltage (VB1) applied to bridge 1. Similarly, and AND logic circuit 257 has two input lines 192 and 262; line 192 is connected to the generator 171 where it is supplied with the aforesaid enable 2 signal, and line 262 is connected to the output of a second comparator 264. The output signal of the circuit 257 will be 1 (and the signal on line 254 will therefore be 0) only when there are concurrent 1 signals on both the lines 192 and 262, and consequently a 0 signal is maintained on the input line 231 of the logic means 230 so long as the second comparator 264 provides a 1 output on the line 262 during "negative" half cycles of VB1.

It will now be apparent that if the output of the first comparator 263 were low or 0 during a positive half cycle of VB1, or if the output of the second comparator 264 were low or 0 during a negative half cycle of VB1, the protective circuit 233 would provide a 1 signal on line 231, in which event the signal on the input line 225 of the logic means 220 would be 0 and normal operation of the one shot 218 would be prevented. In other words, the protective circuit 233 is effective to inhibit the production of β trigger signals by the β logic circuit 174 whenever the first comparator 263 has a 0 output during positive half cycles of VB1 and whenever the second comparator 264 has 0 output during negative half cycles of VB1.

Both of the comparators 263 and 264 have negative input terminals connected by way of a line 266 to the output of an operational amplifier 265 which derives, on the line 266, a reference signal having a value proportional to the instantaneous magnitude of the load current being conducted by the main thyristors in bridge 1 minus a predetermined constant number (K). As is shown in FIG. 4, the amplifier 265 receives its input from a summing point that is coupled to the line 33 and to the slider of a potentiometer 267 connected between a -15 volts d-c control power terminal and ground. The line 33 supplies the summing point with the variable current feedback signal I from the maximum value selector 32 (FIG. 1B). The potentiometer 267 supplies the summing point with a constant bias signal which is negative with respect to ground. The magnitude of K is determined by the setting of this potentiometer, and the magnitude of load current must exceed K in order for the amplifier 265 to provide a relatively positive reference signal on the line 266.

The positive input terminal of the first comparator 263 is connected to the output line 268 of a component 270 which in turn is coupled via lines 271 and 272 to the commutating capacitor 54 (FIG. 1). The component 270 provides on line 268 a voltage feedback signal having a magnitude proportional to the magnitude of the voltage V4 across the commutating capacitor. This signal is positive with respect to ground when the potential at the juncture of the auxiliary rectifiers T4a and T4b is negative with respect to the line 50 (FIG. 1), and it is negative with respect to ground when the potential at the same juncture is positive with respect to the line 50. The voltage feedback signal on line 268 is supplied directly to the first comparator 263, and it is also supplied via an inverting amplifier 273 of unity gain to the second comparator 264. Assuming that the reference signal on line 266 is positive with respect to ground, the first comparator 263 provides a 0 signal on line 261 whenever the voltage feedback signal on line 268 is positive and its magnitude is less than that of the reference signal, and the second comparator 264 provides a 0 signal on line 262 whenever the voltage feedback signal is negative and its magnitude is less than that of the reference signal. The voltage feedback signal will be less than the reference signal if and when the magnitude of V4 is lower than a critical level which varies with the magnitude of load current. The gain of the amplifier 265 and the magnitude of the bias signal at the slider of potentiometer 267 are selected so that the aforesaid critical level is slightly above a level at which V4 becomes insufficient to ensure successful forced commutation of the load current. So long as V4 remains below the critical level, the signal on line 261 will be 0 throughout each positive half cycle of VB1, and the signal on line 262 will be 0 throughout each negative half cycle of VB1. In this event the protective circuit 233 is effective to provide a 1 signal on the line 231, and a 0 signal is consequently maintained on the input line 225 of the logic means 220. As a result, normal thyristor turn off operation of the β logic circuit 174 is desirably blocked or inhibited. This ensures that forced commutation is not attempted under marginal conditions when, because of low voltage across the commutating capacitor, the duration of the circuit turn off interval (T_(OFF) in FIG. 5) might become shorter than the specified turn off time (e.g., the interval 275 in FIG. 5) of the main thyristors in bridge 1.

As was previously explained, a 0 signal on line 225 prevents the logic means 220 from passing to the input line 217 of the one shot 218 any 0-to-1 signal change on the output line 216 of the comparator 215. During each half cycle of normal operation the one shot 218 will actually produce a β trigger signal on line 175 in response to either the 0-to-1 change of the signal on line 216 or the 0-to-1 change of the signal on line 225, whichever occurs later. If α₁ exceeds the desired β, the 0-to-1 change on line 225 is later, and consequently the β trigger signal is produced by the one shot 218 immediately upon termination of the interval during which there is a 1 signal on output line 232 of the time delay circuit 235. In this event the actual extinction angle is greater than desired. When β exceeds α₁ by a sufficient margin, the 0-to-1 change on line 216 is later and the β trigger signal coincides with this change. In either case, the generation of firing signals for the auxiliary controlled rectifiers in the forced commutation subcircuit 4 of bridge 1 is properly coordinated with the generation of firing signals for the main thyristors in the same bridge.

The β logic circuit 174 includes means 247 responsive to the frequency of the β trigger signals on line 175 for providing a signal "β OK" on line 70. The frequency responsive means 247 is suitably constructed and arranged so that β OK is 1 except when the one shot device 218 is operating normally in response to periodic 0-to-1 signal changes on line 217 to produce a train of β trigger signals having a frequency higher than a predetermined magnitude (e.g., approximately 1.4 times the frequency of V1). The line 70 is connected to the firing reference generator 65 (FIG. 2), and whenever the forced commutation subcircuit is operating normally the resulting 0 signal on this line is effective to disable clamping means (not shown) that imposes a predetermined minimum limit on α₁.

While a preferred embodiment of the invention has been shown and described by way of illustration, various modifications thereof will probably occur to persons skilled in the art. For example, instead of being proportional to the highest one of the individual motor currents, the current feedback signal I that is supplied to the protective circuit 233 could be derived from the sum of the currents in the three motors M1, M2, and M3. It is therefore intended by the concluding claims to cover all such changes and modifications as fall within the true spirit and scope of this invention. 

We claim:
 1. Improved means for controlling an electric power converter used to supply direct current to an electric load circuit, said converter comprising the combination ofan electric rectifying circuit having a-c input terminals, a pair of d-c output terminals, means including at least one pair of alternately conducting controllable electric valves for interconnecting said input and output terminals so as to convert alternating voltage at said input terminals to direct voltage of variable average magnitude at said output terminals, and a cyclically operative forced commutation subcircuit including at least one pair of auxiliary controlled rectifiers respectively connected, in circuit with a commutating capacitor, across said controllable valves, whereby each of said valves in turn is forced to turn off upon turn on of a complementary one of said auxiliary rectifiers, means for applying alternating voltage from an a-c power source to the input terminals of said rectifying circuit, means for connecting the output terminals of said rectifying circuit to said load circuit, means for providing a voltage feedback signal representative of the voltage across said commutating capacitor, means for providing a current feedback signal representative of the instantaneous magnitude of direct current supplied to said load circuit, and control means for generating a set of firing signals to periodically turn on said controllable valves in synchronism with said alternating voltage at an ignition angle that can be advanced or retarded as desired and for normally producing periodic trigger signals for turning on said auxiliary rectifiers whereby said valves are cyclically turned off in synchronism with said alternating voltage at a desired extinction angle which varies within predetermined limits, wherein the improvement comprises protective means associated with said control means and responsive to said feedback signals for inhibiting the production of said trigger signals when the magnitude of said capacitor voltage is lower than a critical level which is determined by the magnitude of said current.
 2. The improvement of claim 1 wherein said protective means comprises means supplied with said current feedback signal for deriving a reference signal having a value proportional to the magnitude of said current minus a predetermined constant number, and means supplied with said reference signal and with said voltage feedback signal for preventing said control means from producing said trigger signals whenever the magnitude of said voltage feedback signal is less than the magnitude of said reference signal. 